Networking - Linux gateway to internet for Mac

[Marshal Wong <masee@usa.net>]

Hello Everyone!

I have a couple of questions.  (Of course I have a few questions.  Why else
would I be posting. :) )

1.  I'm networking together a iMac and my Linux box for a local
    network.  I've managed to get the two computers talking to each
    other now with netatalk and macgate.  I don't know how well they
    are talking to each other, but at least file sharing is
    happening.  What I'd like to do is to set the network up so that
    the Mac can use the linux box as a gateway to the internet.  We
    only have one phone line for the computer and the iMac doesn't
    have a phone out jack, so I can't daisy chain the modems
    together.  I'm a complete newbie at networking, so if someone
    could give me a run down on more or less what to do, or what
    manuals to read, I'd greatly appreciate it.

2.  Along the same lines, I'd like to harden the machine against the
    outside world a bit.  I know I probably should put up a firewall,
    but have no clue as to where to start.  Again any hints would be
    great.

3.  A bit off topic, but has anyone had any expriences installing
    GNU/Linux on IBM Aptivas?  Any show stoppers that I might have to know
    about?  I have to install GNU/Linux onto a professors computer
    Monday, and was just wondering how much patience to take along.  :)

I think that's it for now.  Thanks in advance.

Marshal